1. Field of the Invention
This invention relates to rate responsive pacemakers and, more particularly, to rate responsive pacemakers that combine the outputs of both an activity sensor and a metabolic demand sensor in order to respond promptly, during exercise, to changes in a patient's metabolic demand.
2. Description of the Prior Art
Many attempts have been made to control the heart rate of a pacemaker patient so that it will duplicate the intrinsic heart rate of a healthy person both when the patient is at rest and when the patient is involved in various levels of exercise (i.e., so that the pacemaker will be truly rate responsive). Metabolic-related parameters heretofore proposed for controlling the pacing rate include the QT interval, respiration rate, venous oxygen saturation, stroke volume, venous blood temperature, and minute volume, among others. In addition, the use of mechanical and electrical sensors which detect patient motion have also been explored in attempts at achieving improved rate-responsiveness.
However, the metabolically-related parameters used for controlling rate responsive pacemakers tend to react slowly in reflecting changes in the patient's level of exercise. This can result in the patient having a hemodynamic deficiency due to the lag time involved between the onset of a new level of exercise and the reaction thereto by the pacemaker.
Mechanical/electrical motion sensors, on the other hand, respond quite rapidly to changes in the patient's level of exercise. However, they frequently provide an erroneous level of activity due to movement caused by other types of activities such as travel in a vehicle.
More recently, attempts have been made to develop dual-sensor rate responsive pacemakers in order to compensate for deficiencies found in earlier rate-responsive pacemakers making use of a single sensor. An example of such a dual-sensor rate-responsive pacemaker may be seen in U.S. Pat. No. 4,782,836 to E. Alt, entitled "Rate Adaptive Cardiac Pacemaker Responsive To Patient Activity And Temperature" and which issued on Nov. 8, 1988. This patent discloses the use of an activity (motion) sensor in conjunction with a temperature sensor. The patent employs two algorithms relating blood temperature to pacing rate, one for an inactive condition of the patient, and another for an active condition of the patient. The motion sensor selects one or the other of the algorithms depending on the activity level of the patient.
U.S. Pat. No. 4,860,751 to F. J. Callaghan, entitled "Activity Sensor For Pacemaker Control", and which issued Aug. 29, 1989, also relates to a dual-sensor rate-responsive pacemaker. In this patent, a cardiac pacer is provided with both an activity sensor and a physiological sensor. The output of the activity sensor is utilized by control circuitry to enable the physiological sensor to monitor a selected physiological parameter only if the physical activity of the patient exceeds a selected threshold. This allows the electrical energy requirements of the pacer to be reduced.
U.S. Pat. No. 4,905,697 to K. S. Heggs et al., entitled "Temperature-Controlled Cardiac Pacemaker Responsive To Body Motion", and which issued Mar. 6, 1990, discloses the use of a motion sensor to cause the pacing rate to decrease upon cessation of exercise, after there has been an increase in the pacing rate, based on sensed blood temperature changes.
U.S. Pat. No. 4,926,863 to E. Alt, entitled "Rate Responsive Cardiac Pacemaker", and which issued May 22, 1990, discloses a dual-sensor rate-responsive pacemaker that employs activity sensing and temperature sensing. An accelerometer is used to sense physical activity of the patient. The activity sensor converts mechanical movement of the patient to a corresponding electrical signal, and only a portion of the electrical signal, in a frequency range below 4 Hz, is used to discriminate against signal components (e.g., environmental noise) arising from other than the patient's physical activity. This signal is combined with a sensed physiological parameter signal to confirm the metabolic state of the patient.
U.S. Pat. No. 5,101,824 to A. Lekholm, entitled "Rate-Responsive Pacemaker With Circuitry For Processing Multiple Sensor Inputs", and which issued Apr. 7, 1992, discloses a rate-responsive pacemaker employing two or more sensors that are indicative of metabolic demand to realize the advantages of both sensors in a circuit producing a rate command signal. This signal is used to operate the pacer at an optimum pacing rate to match the patient's physiological need. An addressable rate matrix, which is used to produce a specific rate unique to each combination of sensor inputs measured at a particular time, is employed in this patent. The sensor inputs include activity as well as a number of other physiological parameters.
U.S. Pat. No. 5,197,467 to B. M. Steinhaus et al., entitled, "Multiple Parameter Rate-Responsive Cardiac Stimulation Apparatus", and which issued Mar. 30, 1993, discloses a rate responsive pacemaker which uses a single impedance measuring circuit to sense multiple parameters for rate adaptation. The impedance-measuring circuit employs a measuring current having frequency components which may be varied in order to control the selection of the physiological parameters that are to be sensed, and the apparatus analyzes the physiological parameters and determines the best pacing rate to be selected based on such analysis.